1. Field of the Invention
The present invention relates to a spin polarized electron semiconductor source and an apparatus utilizing the same, and more particularly, to improvement of spin polarization and quantum efficiency in the spin polarized electron semiconductor source.
2. Description of the Related Art
The charged weak bosons couple only chirality-left quarks and leptons. In high energy interactions of massless limit, the chirality equals the helicity. A polarized electron beam, therefore, can control weak interactions in high energy experiments, and is expected to play important roles in experiments of e.sup.+ e.sup.- linear colliders. In the experiments using polarized electron beams, in most cases, the sensitivities of the experiments increase proportionally to the square of the spin polarization degree. Therefore, a spin polarized electron source as a photocathode by which electrons having as high spin polarization as possible can be extracted is urgently required. Besides the degree of spin polarization, the amount of charge which can be extracted from the spin polarized electron source is important in the collider. Therefore, it is desirable for a spin polarized electron semiconductor source to satisfy a spin polarization close to 100% for electrons having an aligned spin and a high quantum efficiency for a large current.
As such a spin polarized electron source for extracting spin polarized electrons, an example in which the band structure of a bulk semiconductor is utilized is described in a paper (Solid State Communication, Vol. 16, p. 877, 1975) by G. Lanpel et al. In this spin polarized electron semiconductor source, a multilayer in which Cesium (Cs) layers and oxygen (O) layers are alternately laminated is deposited on the surface of a p-type GaAs semiconductor to produce a negative electron affinity. Electrons having a maximum spin polarization of 50% can be extracted from the semiconductor surface by irradiating it with a circularly polarized laser beam having an energy substantially equal to the forbidden band of GaAs. In the band structure of the GaAs semiconductor, a band for heavy holes and a band for light holes are degenerated in the valence band and therefore the ratio of electrons having a downward spin to electrons having an upward spin is 3:1 because of the difference in transition probability when electrons are excited from these bands to the conduction band. For this reason, the maximum polarization of 50% can be obtained.
In order to obtain a further higher polarization close to 100%, it is necessary to remove the degeneracy of the heavy hole band and the light hole band in the valence band. For this purpose, spin polarized electron sources utilizing strained crystal or a short period of a semiconductor superlattice structure are proposed.
As an example of a spin polarized electron semiconductor source utilizing strained crystal there is the paper (Physics Letters A., Vol. 158, p. 345, 1991) by T. Nakanishi et al. FIG. 1A shows the structure of such a spin polarized electron semiconductor source utilizing strained crystal. In the example, a lattice relaxation layer 102 of p-type GaPAs which has a lattice constant greater than that of a p-type GaAs substrate 101 and no lattice relaxation is provided on the substrate 101, and a thin strained layer 103 of p-type GaAs in which lattice relaxation is not generated is provided on the lattice relaxation layer 102. A compressive stress acts due to the strain in a direction along the plane in the uppermost GaAs strained layer 103 to align the lattice of the strained layer 103 with the lattice of the relaxation layer 102. As a result, the degeneracy of the heavy hole band and the light hole band is removed in the valence band so that the heavy hole band is positioned higher in energy than the light hole band. Therefore, if the energy of exciting light is chosen to be equal to the energy from the heavy hole band to the conduction band, i.e., a forbidden band energy, electrons are excited only from the heavy hole band so that electrons having completely aligned spin can be obtained. In this manner, spin polarization of 100% ought to be achieved in theory. However, the spin polarization of extracted electrons is lower than 100% in reality because of extension of bands by thermal energy and spin scattering in the strained crystal. As a result of an experiment in which the spin polarization of electrons extracted from the surface of an alternate lamination multilayer of Cs and O formed on the strained layer 103 is measured, a high polarization of 80% or above was obtained.
On the other hand, an example of a spin polarized electron semiconductor source device using a superlattice structure of a short period is described in, for example, a paper (Physical Review Letters, Vol. 67, p. 3294, 1991) by Omori el at. The structure of the electron semiconductor source device using the superlattice structure is shown in FIG. 1B. On a substrate 101 of p-type GaAs, there are sequentially formed a buffer layer 104 of p-type GaAs and a block layer 105 of p-type AlGaAs having a wide forbidden band. The buffer layer 104 is formed to provide a flat surface and the block layer 105 is formed to prevent electrons excited in the substrate 101 from going into a superlattice structure 110. The superlattice structure 110 having a short period is formed on the block layer 105. In the superlattice structure, a well layer 112 of p-type GaAs having a thickness equal to or shorter than a wavelength of electron wave and a barrier layer 114 of p-type AlGaAs having a thickness through which an electron can transmit due to the tunnel effect are alternately laminated. A protection layer 120 of As is formed on the superlattice structure 110. In this case, the degeneracy for heavy holes and light holes is removed in the superlattice structure 110 and a mini band for the heavy holes and a mini band for the light holes are formed in the valence band due to quantum effect. These mini bands occupy different energy levels because of a great difference in effective mass. As a result, similar to the case of the strained crystal, the mini band for the heavy holes takes a position higher in energy than that of the mini band for the light holes. Accordingly, if exciting light is chosen to have the energy from the mini band for the heavy holes to a conduction band and is irradiated to the semiconductor source with a circular polarization, electrons can be excited only from the mini band for the heavy holes and can have completely aligned spins. Therefore, electrons having spin polarization of 100% ought be obtained in theory. As a result of are experiment in which spin polarization of electrons extracted from the surface of a device in which a CsO multilayer was laminated on the surface was measured, a high spin polarization over 70% was obtained.
As described above, when that the GaAs non-strained crystal or the superlattice structure is used as in the conventional spin polarized semiconductor electron source, although the quantum efficiency is relatively high, the spin polarization is insufficient. On the other hand, when the strained crystal is used for the spin polarized electron semiconductor source, although a great polarization is obtained, the quantum efficiency is as low as 0.5% or below because there are defects in the crystal due to the doped impurity and the strained crystal layer cannot be made thicker. In this manner, a high spin polarization and a high quantum efficiency could not be both satisfied simultaneously.